The invention relates to an optical scanning device for scanning an information layer of an optically readable record carrier, which scanning device comprises a radiation source and an objective system with an optical axis for focusing a radiation beam, supplied in operation by the radiation source, to a scanning spot on the information layer, the objective system being provided with a first lens comprising a part of a first, substantially spherical lens body with a first center and a first diameter, and a second lens comprising a part of a second, substantially spherical lens body with a second center and a second diameter which is smaller than the first diameter, the first and the second center being situated substantially on the optical axis.
The invention also relates to an objective system comprising such a first and a second lens.
The invention further relates to an optical player provided with a table which is movable in a given direction of movement, an optical scanning device for scanning an information layer of an optically readable record carrier which can be placed on the table, and a displacement device with which at least an objective system of the scanning device is movable in a direction substantially perpendicular to the direction of movement of the table.
An optical scanning device, a player in which the optical scanning device is used, and an objective system used in the scanning device of the types described in the opening paragraphs are known from EP-A 0 944 049. The first lens of the known objective system is an objective lens and the second lens is an auxiliary lens which is arranged between the objective lens and the record carrier to be scanned. By using the second lens, the known lens system has a relatively large numerical aperture so that a relatively small scanning spot is provided on the record carrier to be scanned. The second lens comprises more than half of the second spherical lens body in order to form the scanning spot at a distance r/n from the second center in the direction of the record carrier, in which r is the radius of the second lens body and n is the refractive index of the material of the second lens body. This displacement of the position of the scanning spot yields a further increase of the numerical aperture. Thus, the known scanning device is suitable for scanning record carriers having relatively small elementary information characteristics, i.e. record carriers having a relatively large information density such as, for example, a high-density CD. Since the objective system has a free working distance which is smaller than the wavelength of the radiation, the lens bodies and their mutual positions have relatively wide tolerances.
A drawback of the known objective system, the known optical scanning device and the known optical player is that the desired optical quality of the objective system is only achieved with relatively great trouble in spite of said wide tolerances. At a larger free working distance, the tolerances become considerably stricter and the desired optical quality is even more difficult to achieve.
It is an object of the invention to provide an objective system, an optical scanning device and an optical player of the types described in the opening paragraphs, in which the desired optical quality can be achieved with less trouble.
To achieve this object, a scanning device according to the invention is characterized in that the first and the second lens comprise more than one half of the first and the second spherical lens body, respectively. The invention is based on the recognition that the optical quality of an objective system with two lenses is greatly dependent on the mutual positioning of the two lenses. The optical axes of the separate lenses should coincide within predetermined relatively small tolerances. Viewed in directions perpendicular to the optical axis of the objective system, the optical axes of the lenses should therefore have accurate positions with respect to each other and should accurately be parallel to each other. The alignment of the known objective system is relatively difficult because the provision of positioning faces on the two lenses with the desired accuracy is a very elaborate process. However, when a lens comprises more than half a spherical lens body, the ball zone, i.e. the outer side of the lens body is found to form a very satisfactory positioning face around the large circle perpendicular to the optical axis. Moreover, spherical lens bodies can be manufactured in a very accurate way by means of relatively simple processes such as, for example, a rolling process. Due to the spherical shape of the lens, the position of the outer side of the lens body with respect to the center is very well defined, so that the desired accuracy of the mutual positions can be realized in a relatively easy way.
An additional advantage of such a lens body is that, after positioning in a fitting, the lens is still tiltable in the fitting through limited angles around the center of the lens body without the position of the lens, viewed perpendicularly to the centerline of the fitting, being influenced. The direction of the optical axis of the lens, passing through the center of the lens body, is thereby adjustable. The edge of the lens body where the spherical shape merges into a relatively flat portion is relatively rough due to the manufacture of the flat portion, for example due to grinding and polishing of the flat portion. When, during the process of joining the lens system, the rough edge of the lens body comes in contact with the inner wall of the fitting, or when its rough edge engages the inner wall after the process of joining, it is possible that the lens body will get out of center or even gets stuck. The lens bodies according to the invention do not have this problem because the rough edge is remote from the positioning face engaging the inner wall.
When both the first and the second lens of the objective system comprise more than half of a spherical lens body, the centers of the first and second lens can be positioned with great accuracy on the centerline of the fitting or fittings, and, moreover, the optical axes of the first and the second lens can be accurately rendered parallel to each other. Due to the relatively simple alignment of the first and the second lens, the desired optical quality of the objective system can be achieved with relatively little trouble.
The thickness on the optical axis of the first and the second spherical body is preferably larger than 1.01 times the radius of the spherical body. In that case, the engagement of the lens body with the inner wall of the fitting remains satisfactory when, during the process of joining the lens system, the optical axis of one of the lenses extends at an angle of 0.5xc2x0 to the centerline of the fitting. The difference between the thickness and the radius of the lens body is preferably larger than the manufacturing tolerance of the thickness.
A particular embodiment of a scanning device according to the invention is characterized in that at least one of the first and the second lens is bounded on one side by a boundary face extending perpendicularly to the optical axis. A spherical lens body can be provided relatively easily with an accurately positioned, plane boundary face within narrow tolerances. The boundary face should be plane only in an accurate way, with the optical axis of the lens being formed by the centerline of the lens body which is perpendicular to the boundary face. Such a lens can be aligned easily and accurately on an abutment of a fitting. When the abutment extends in a plane perpendicular to the centerline of the fitting, the optical axis of the lens will be accurately parallel to the centerline. Said abutment should be provided perpendicularly to the centerline of the fitting in an accurate manner, which can be done in a relatively simple manner by means of conventional tools such as, for example, a lathe. When both the first and the second lens are provided with a boundary face extending perpendicularly to the optical axis, it is relatively easy to render the optical axes of the two lenses substantially parallel.
The first and the second lens may be fixed in a first and a second fitting, respectively, which can be displaced with respect to each other in the direction of the optical axis. When the two fittings are accurately aligned with respect to each other, the optical axes of the lenses will also coincide accurately. A particular embodiment of a scanning device according to the invention is, however, characterized in that the first and the second lens are secured in a fixed position with respect to each other. Consequently, both lenses can be fixed in a single lens holder comprising a first and a second fitting for the first and the second lens, respectively. It is relatively simple to cause the centerline of the first fitting and the centerline of the second fitting in such a lens holder to coincide within small tolerances. The inner walls of the fittings may be provided with projecting ribs engaging the spherical outer sides of the lens bodies. However, the inner walls are preferably substantially circular-cylindrical with a common centerline. This can be realized in a relatively simple manner by providing the first and the second fitting of the lens holder with the first and the second circular-cylindrical inner wall, respectively, by means of conventional tools such as, for example, a lathe.
A further embodiment of a scanning device according to the invention is characterized in that one side of at least one of the first and the second lens has an aspherical shape. The aspherical shape provides sufficient freedom of design for realizing the strict requirements imposed on the objective system by a scanning device for optically readable record carriers. The aspherical shape may be obtained by means of a lacquer layer on the boundary face or the spherical surface of the lens by means of a known and conventional replica process.
A particular embodiment of a scanning device according to the invention is characterized in that at least one of the first and the second lens body comprises a transparent material having an Abbe number which is larger than 63. Due to the high Abbe number, the wavelength dependence of the focal length of the objective system can be reduced in such a way that the wavelength variation during switching between write and read power of a semiconductor laser leads to an acceptably small defocusing.
The objective system can be manufactured at lower cost when the first and the second lens body comprise the same transparent material. The number of degrees of freedom of the objective system is then still sufficiently large to realize the desired optical properties.
A particular embodiment of a scanning device according to the invention is characterized in that at least one of the first and the second lens body comprises a transparent material having a refractive index which is smaller than 1.54. A relatively low refractive index of the material of the lens body leads to a lens body having a relatively small thickness on the optical axis and therefore a relatively small mass. A lighter lens body yields an increase of the bandwidth of the actuators which are used to keep the scanning spot formed by the objective system in focus and on the track. Moreover, materials having a low refractive index are less expensive than materials having a high refractive index.
A special embodiment of an optical scanning device according to the invention is suitable for writing and erasing magneto-optical record carriers and is provided with a magnet coil for this purpose. The magnet coil is preferably arranged on the second boundary face of the second lens. The thickness on the optical axis of the second lens is preferably larger than half the diameter of the lens body so as to obtain the advantages of positioning according to the invention.
In a special embodiment of a scanning device according to the invention, the magnet coil is countersunk in the second lens so as to reduce the total thickness of the lens with the magnet coil. To this end, the second boundary face has a central boundary face and a peripheral boundary face shifted along the optical axis, the magnet coil is arranged on the peripheral boundary face, and the part of the second lens which is bounded by the plane of the peripheral boundary face comprises more than half of the second spherical lens body. The central boundary face constitutes an end face of a portion of the second lens situated on the optical axis, which portion projects from the plane of the peripheral boundary face and is surrounded by the magnet coil. The thickness on the optical axis of the second lens is the thickness as is required for the optical design of the objective system. The diameter of the projecting portion is sufficiently large to pass the converging radiation beam through the lens without vignetting.
A particular embodiment of a scanning device according to the invention is provided with a lens holder comprising a fitting for the second lens with a centerline and an inner diameter, the centerline substantially coinciding with the optical axis, and the fitting extending along the optical axis in a direction from the first to the second lens up to beyond the center of the second lens body, the magnet coil being secured on a side of the fitting remote from the first lens and having an inner diameter which is substantially equal to the inner diameter of the fitting. The second lens may be positioned on the well-defined inner side of the fitting, while the less well-defined inner side of the magnet coil does not have any influence on the position of the second lens. Due to the positioning directly around the second lens, the magnet coil has a relatively small inner diameter so that a relatively large magnetic field is generated.
An objective system according to the invention is characterized in that the first and the second lens comprise more than one half of the first and the second spherical lens body, respectively.
An optical player according to the invention is characterized in that the optical scanning device is an optical scanning device as described hereinbefore.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.